In the field of graphic arts and medical treatment, there have been concerns in processing of photographic film with respect to effluents produced from wet-processing of image forming materials, and recently, reduction of the processing effluent is strongly demanded in terms of environmental protection and saving space. There has been desired a photothermographic material for photographic use, capable of forming distinct black images exhibiting high sharpness, enabling efficient exposure by means of a laser imager or a laser image setter.
Thermally developable photothermographic materials are disclosed, for example, in D. Morgan and B. Shely, U.S. Pat. Nos. 3,152,904 and 3,457,075, and D. H. Klosterboer, “Thermally Processed Silver Systems” (Imaging Processes and Materials, Neblette, 8th Edition, edited by J. M. Sturge, V. Walworth, and A. Shepp, page 2, 1969), etc. Such photothermographic materials contain a reducible light-insensitive silver source (such as organic silver salts), a catalytically active amount of photocatalyst (such as silver halide) and a reducing agent, which are dispersed in a binder matrix. Such photothermographic materials are stable at ordinary temperature and forms silver upon heating, after exposure, at a relatively high temperature through an oxidation-reduction reaction between the reducible silver source (which functions as an oxidizing agent) and the reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image produced by exposure. Silver formed through reaction of the reducible silver salt in exposed areas produces a black image, which contrasts with non-exposed areas, leading to image formation.
Such photothermographic materials, for example, are cut to a given width and wound onto a roll core to form a rolled photothermographic material (photothermographic material in roll). Then, a light-shielding leader of a relatively low moisture permeability is attached to the top of the photothermographic material in roll and wound around the circumferential of the photothermographic material in roll, which is further packed with a light-shielding material to form a room-light loading type photothermographic material package that can be loaded into an image recording device under room-light (hereinafter, also denoted simply as a package).
Since organic silver salts are employed as a silver source in the photothermographic material, in cases when moisture is contained in the photothermographic material, the moisture promotes dissociation of silver ions from the organic silver salt, causing reaction with the reducing agent to produce fog during storage of raw photothermographic material and resulting in contrast variation.
In order to enhance pre-exposure storage stability of a photothermographic material, various countermeasures have been taken in both photothermographic materials and a packing methods thereof. As one countermeasure from the photothermographic material side, for example, JP-A No. 6-301140 (hereinafter, the term, JP-A refers to a Japanese Patent Application Publication) discloses a technique of keeping the residual solvent content after coating at a prescribed level, thereby minimizing aging variations in developing temperature or density; JP-A No. 2000-310830 discloses a photothermographic material, in which a support exhibiting an equilibrium moisture content at 60% RH of 0.5 wt % or less and further thereon, a coating solution having a water content of 2 wt % or less is coated and dried so that the dried photothermographic material exhibits a residual solvent content of 2.0 wt % or less; and JP-A No. 11-352623 discloses a technique of packaging at 20 to 60% RH. There is also disclosed in JP-A No. 2000-206653 a technique of packing photothermographic materials with packing material exhibiting an oxygen permeability of 50 ml/atm·m2·25° C.·day or less and a moisture permeability of 10 g/atm·m2·25° C.·day or less.
However, it is a proven fact that reducing the solvent content of photothermographic material or packing photothermographic material with a packing material exhibiting a low moisture permeability under a relatively low humidity is insufficient to prevent fogging or variation in image quality, caused during storage of raw photothermographic material. In fact, complicated and troublesome controls, such as refrigerated storage or storage in an atmosphere maintained at a given humidity have been conducted to maintain photographic performance following storage of pre-exposure photothermographic material. Accordingly, there is desired development of a photothermographic material having no necessity of such troublesome control and exhibiting superior raw stock stability, and a package thereof.